Writers explaining what happened in the book 1 in book 2, why?

This is a choice that some writers/editors make to varying degrees. Sometimes it’s a sentence here or there and other writers have paragraphs in the 4 book in a series explaining a lot of the previous books. Why do this at all? It takes me out of the story. Who could possibly be skipping book one of a trilogy? Sometimes it seems to be to the extent that that is clearly for people who skipped book 1 because some of the book 1 reminders were a major part of book 1. Like, “they were childhood friends…” yeah the whole book 1 was about that! It is this degree of not reminders but explaining other books that I cannot wrap my head around why this decision is made.

Anyone?

I can imagine this is a constant argument between writers and editors. One wants them one doesn’t. What is the reason this wins out in the end and explaining previous books is left in?